The work proposed in the ensuing pages develops new techniques for the annelation of alkenes utilizing a photochemical 2 plus 2 cycloaddition as the key step. The annelating agents are 1,3-dioxolen-4-ones, or their synthetic equivalents, and the products after elaboration are substituted cyclohexenones. Syntheses of a number of sesquiterpenes are proposed as are several alkaloids. An efficient scheme is also developed for the preparation of delta-lactones, and the consequent synthesis of several iridoid terpenes such as plagiolactone and genipic acid. Finally, utilization of the cyclobutane formed in the initial photoreaction as a precursor of 1,3-dienes, gamma-lactones, and cyclopentenones is proposed.